Oxidation catalysts containing manganese and a polyvalent metal, such as copper, cobalt, or silver, have been in commercial use for many years. They are referred to as "Hopcalites". The Hopcalite catalyst containing copper with manganese has been commonly used in gas masks for oxidation of carbon monoxide. Hopcalites may be formed by the coprecipitation of Mn(OH).sub.2 with Cu(OH).sub.2 or other polyvalent metal hydroxide. (See, for example, U.S. Pat. No. 1,345,323 of 1920.) Manganese catalysts containing copper or other polyvalent metal have also been prepared from a solution of the metal salt and potassium permanganate in the presence of hydrogen peroxide. U.S. Pat. No. 1,971,168 (1934), and U.S. Pat. No. 1,995,353 (1935). The resulting catalyst is described as an acid manganite of the copper or other polyvalent metal, such as, for example, Cu(HMnO.sub.3).sub.2. As far as is known, such acid manganite catalysts have not found commercial applications.
Dodman et al in U.S. Pat. No. 3,700,605 (1972) describe a procedure for preparing a manganese catalyst by first forming a manganese oxide precipitate from potassium permanganate in the presence of alkali metal hydroxide. The recovered precipitate is resuspended in water, and an aqueous solution of a salt of a polyvalent metal is introduced into the suspension to react therewith. The patent recommends the use of cobalt, copper, silver or lead ions to replace the alkali metal in the precipitate. This ion exchange reaction is described as being carried out at ambient temperature (e.g. 20.degree. C.). No pH adjustment is disclosed. Dodman et al propose that an inert insoluble substance (e.g. kieselguhr or silica) should be suspended in the potassium permanganate solution as it is converted to the manganese dioxide precipitate, thereby providing a catalyst support. The catalyst is used for reduction reactions, such as the reduction of nitrocompounds with carbon monoxide.
Dodman et al did not propose the use of "by-product" manganese dioxide as a starting material for catalyst preparation. However, the by-product manganese dioxide resulting from the use of potassium permanganate to oxidize organic substances under alkaline conditions is similar to the manganese dioxide precipitate prepared by Dodman et al. Chemically, they are both alkali metal delta manganese dioxide hydrate.
In the procedure of Dodman et al, the alkali metal manganese dioxide precipitate (the delta manganese dioxide hydrate) is suspended in water, and the aqueous solution of the heavy metal salt is gradually introduced into the suspension. The ion exchange reaction therefore starts at a high alkaline pH, and most if not all of the reaction takes place under alkaline pH conditions. During the course of the experimental work leading to the present invention, it was discovered that the Dodman et al reaction procedure is undesirable. At start of the reaction, under Dodman's pH conditions part of the heavy metal ions form hydroxide precipitates, which reduce the reaction rate, require more reagent, and, if not redissolved or extracted become associated with the catalyst product.